Fe-Cr Alloys Research Articles

Fuel-clad interactions: Uranium nitride and uranium dioxide with FeCrAl and Ti-modified stainless steel

This study elucidates on the compatibility of fuel and clad materials for potential use in lead cooled fast reactors. Diffusion couples were performed on representative, unirradiated fuel forms (UN and UO2) with potential cladding materials, a Ti-modified stainless steel (D9) and a FeCrAl alloy (Kanthal APMT). Temperatures of 800–1200°C, with durations up to 100 h, were investigated. Chemical interaction between all combinations of the fuel and cladding candidates was found to be minimal, with no diffusion gradients observed or extensive interface phases formed. When interaction occurred, it was in the production of a minor discrete phase. At the highest temperatures, D9 with UN resulted in the formation of a Ti-C-N phase. D9 with UO2 resulted in the formation of a Ti-C-O phase. FeCrAl with UN resulted in the formation of an Al-N phase. FeCrAl with UO2 results in the formation of an Al-O phase. Despite the observed mechanical bonding between fuel and cladding material, no evidence of any chemical interaction with the uranium in the fuel material was found. At the higher temperatures investigated (>1000°C) there was evidence of clad softening resulting in transfer of material to the fuel surface.

Dislocation loops and hardening induced by Au ion irradiation in FeCrAl alloys with three solute concentrations

Three alloys, Fe-9Cr-6Al, Fe-11Cr-6Al and Fe-13Cr-4.5Al, were irradiated with 6 MeV Au+ ions to show their irradiation effect in this work. Statistics of the density and size of dislocation loops were performed in TEM images and 〈100〉 and 1/2 〈111〉 dislocation loops were treated differently. Irradiation hardening was measured by nanoindentation and hardness curves were analyzed by the Nix-Gao model. The DBH hardness was calculated by using statistical data of dislocation loops and lines. The results show a defect evolution of “dots to loops to lines” is clear as the damage dose increases from 5 to 20 dpa, the number and size of dislocation loops are increased with increasing the temperature to 400 °C at 10 dpa, and simultaneously, the irradiation hardening is enhanced. The Nix-Gao bulk equivalent, measured and DBH-calculated hardness curves are basically consistent in trend but decreases sequentially in numerical value. The loops-hardening mechanism is also discussed. Furthermore, an increase of the Cr content tends to reduce the density of dislocation loops, especially for the 400 °C irradiation and the alloy with 13 % Cr indeed show a good irradiation stability.

Effects of Mn concentration on durability of Fe-Cr ferritic steels dispersed with La2O3 for solid oxide fuel cell interconnects

This study investigates the effect of Mn concentration in La2O3-dispersed ferritic steel on the oxidation behavior, area-specific resistance (ASR), and chromium volatilization for solid oxide fuel cell (SOFC) interconnects. The results show that increasing Mn concentrations lead to a gradual increase in the oxidation rate of the alloy samples. The alloy with 0.3 wt% Mn primarily forms Cr2O3 during the initial oxidation stage, whereas the alloys containing more than 1.0 wt% Mn promote the growth of (Mn,Cr)3O4, leading to an accelerated oxidation rate. The ASR value exhibits a decreasing trend initially, followed by an increasing trend, as the Mn concentration in the alloy increases from 0.3 to 2.0 wt%. The alloy with 1.0 wt% Mn shows the lowest ASR value of 12.2 mΩ cm2 after 1000 h. The alloys demonstrate a tendency to reduce chromium volatilization as the Mn concentration increases, which can be attributed to the growth of (Mn,Cr)3O4 at the topmost layer of the oxide. The study suggests that controlling the Mn concentration in the Fe-Cr alloys can help in reducing the ASR values and Cr volatilization, making the ferritic steel alloy with 1.0 wt% Mn an effective choice for SOFC interconnects.

Fe–Cr–Al Alloy Development Via Wire-Feed Additive Manufacturing Technology

Fe–Cr–Al Alloy Development Via Wire-Feed Additive Manufacturing Technology

Initial oxidation of Fe–Cr alloys at different temperatures and atmospheric conditions: A ReaxFF molecular dynamics approach

Initial oxidation of Fe–Cr alloys at different temperatures and atmospheric conditions: A ReaxFF molecular dynamics approach

Effect of molybdenum on the oxidation resistance of FeCrAl alloy in lower temperature (400 °C) and higher temperature (1200 °C) steam environments

The effect of 0, 1, 3 wt% Mo addition on the corrosion behavior of Fe21Cr5.5Al was studied through steam tests at 400 °C and 1200 °C. At 400 °C, Mo addition leads to non-protective Fe-Mo oxide growth on the surface, compromising protective uniform Cr oxide growth and subsequent Al2O3 layers as in the Mo-free specimen. At 1200 °C, the presence of Mo causes the spallation of the surface oxide scale. However, a thin alumina surface film was present. Current results show that Mo addition is detrimental to the corrosion resistance of FeCrAl alloy under both operational and accidental temperature scenarios.

Evaluation of the Corrosion Properties of FeCrAl Alloys for Fuel Rods Cladding in the Entire Fuel Cycle

Because of its resistance to high temperature (>1000°C) attack by steam and good mechanical properties above 400°C, iron-chromium-aluminum alloys (FeCrAl) are considered attractive candidates to replace current zirconium (Zr) based alloys as cladding for light water reactor (LWR) fuels. FeCrAl alloys were sporadically investigated for nuclear applications since the 1950s. Most of the knowledge on these alloys in the literature is for dry environments and temperatures above 1000°C and little information was available for lower temperatures or wet environments. As cladding for LWR, the FeCrAl alloys are expected to survive the nuclear fuel cycle, that it, from manufacturing to used fuel disposal. The presentation will cover the corrosion behavior aspects of FeCrAl from manufacturing using powder metallurgy and welding, to residence in the reactor core for up to ten years (including loss of coolant scenarios), to used fuel residence in cooling pools, to fuel retrievability and reprocessing followed by long term dry cask storage and eventual geologic repository disposal.

An investigation on nitrogen uptake and microstructure of equimolar quaternary FeCoNiCr high entropy alloy after active-screen plasma nitriding

Under nitrogen diffusion treatments, N-expanded austenite (γN) can form at the surface of self-passivating Fe-Cr, Ni-Cr, and Co-Cr alloys at low temperatures, which provides beneficial hardening and enhancements in wear resistance without reducing corrosion resistance. Given the wide research interests in multicomponent equimolar alloys, an equimolar quaternary FeCoNiCr high entropy alloy (HEA) was investigated after active-screen plasma nitriding at 430–480 °C in this study. Firstly, the formation of γN-FeCoNiCr case at 430 °C was demonstrated with the bright case appearance after metallographic etching, the lattice expansion under XRD, the FCC electron diffraction patterns and the shear bands under TEM. Secondly, the thick treatment cases at ∼9–16 μm first indicated that N interstitial diffusion was not sluggish in the FeCoNiCr surface. Thirdly, analogous to stainless steels, the onset of dark regions in the etched γN-FeCoNiCr case was owing to the formation of a cellular mixture of CrN + γ-(Fe, Co, Ni) nano-lamellae at elevated treatment temperatures. The residual bright regions in γN-FeCoNiCr at 480 °C showed ∼1–3 nm CrN nanoprecipitates with no substantial Cr segregation. Additionally, a significant nanocrystalline layer was seen at the topmost surface at 480 °C, which is most likely associated with the high substrate Cr content.

(Invited) Comparative High Temperature Properties of Iron-Chromium-Aluminum and Zirconium Alloys

Light water reactors (LWR) have been using zirconium (Zr)-based alloys for the cladding of the fuel for more than six decades. Zirconium alloys oxidize rapidly in presence of water and steam at temperatures above 400°C and therefore they do not respond well to scenarios of loss of coolant accident conditions. In the 1950s and 1960s iron-chromium-aluminum (FeCrAl) alloys were explored to replace Zr alloys manly because the former has better oxidation resistance and better mechanical properties than the latter. The use of FeCrAl for LWR fuel cladding has been revived in the last decade following the 2011 accident at the Fukushima Daiichi site. This work will show comparative data to illustrate the advantages of FeCrAl in high temperature scenarios considering oxidation resistance, mechanical properties, and interactions with the fuel pellet.

Laser powder bed fusion of oxide dispersion strengthened FeCrAl alloy: Processing and microstructural evolution

Optimizing and regulating the process parameters to form a uniform distribution of nanosized dispersoids remains a major challenge in additive manufacturing oxide dispersion strengthened (ODS) alloys. This study introduces the influence of scanning speed and laser power on the microstructure evolution of laser powder bed fusion (LPBF) ODS FeCrAl alloy. Low laser power and high scanning speed reduce the size of oxide particles and increase their number density. The energy density threshold is 83 J/m3, below which the powder fails to achieve complete melting, leading to the formation of defects during solidification. High density and large number of oxide particles were obtained for the printed samples under the parameters of 140 W laser power and 600 mm/s scanning speed. The average size and number density of oxide particles were ∼ 21.61 nm and ∼ 1.8 × 1020/m3, respectively. High scanning speed have a rapid cooling rate which affects the dislocation density and the average width of the columnar grains. Low laser power significantly weakens the 〈100〉 fiber texture, which is related to the degree of recrystallization in the remelted region. The core-shell oxides consisting of Y-Al-O and Y-Ti-O were distributed in the samples deposited at low and high laser powers, respectively. The formation of core-shell oxide particles in LPBF ODS FeCrAl alloys involves solid-state amorphization and recrystallization. This study can be a guideline to achieve materials with specific microstructure for ODS alloys.

Unexpected deformation-induced martensitic phase transformations in Ni–Cr and Ni–Cr–Fe alloys

Deformation of Ni–Cr and Ni–Cr–Fe alloys has historically been understood to occur through dislocation slip, while deformation-induced martensitic transformations are believed to be suppressed by the high stacking fault energy (SFE). The present study challenges these longstanding beliefs with experimental and theoretical evidence of deformation-induced martensitic transformations in Alloy 625 and Ni–20Cr, respectively. Systematic investigations are conducted along the three principal grain orientations; experiments employ nanoindentation coupled with post mortem transmission electron microscopy (TEM) analysis, while molecular dynamics (MD) simulations are conducted in uniaxial tension. Experimental results reveal γ→ε and γ→ε→α′ martensitic transformations in Alloy 625 consistent with the Bogers-Burgers-Olson-Cohen (BBOC) intersecting shear mode. The orientation relationship is characterized as a distorted Shoji-Nishiyama OR between FCC/HCP and Kurdjumov-Sachs OR between FCC/BCC. By contrast, Alloy 690 does not exhibit martensitic transformations despite having a lower SFE than Alloy 625, due to Mo solute strengthening in Alloy 625. MD simulations reveal consistent transformation mechanisms across grain orientations, but orientation-dependent differences in Schmid factors and critical resolved shear stress affect the strain evolution and extent of the transformation. This study concludes that SFE alone may not comprehensively dictate whether deformation-induced martensitic transformations will occur; other factors such as free energy and strain rate also influence transformability.

Significant suppression of helium bubbles in oxide dispersion strengthened FeCrAl alloys irradiated by high concentration of helium

Oxide dispersion strengthened (ODS) FeCrAl alloys were successfully fabricated by various mechanical alloying time (10h and 40h), combined with spark plasma sintering (SPS) technique. To understand their helium irradiation resistance, two kinds of ODS FeCrAl alloys were implanted with helium ions at 400 keV in the fluence of 1 × 1021/m2 at 350 °C, 400 °C, and 450 °C to investigate the effect of high concentration helium on the materials in advanced nuclear reactors. The distributions of helium bubbles associated with the microstructural features were characterized by transmission electron microscope (TEM). The experimental results showed that both the helium bubble sizes and volume fractions increased with increasing the irradiation temperatures. The high sink strength of nano-oxides and small inter-oxides distance contributed to the good suppression of helium bubbles growth. In particular, we identified that the high-density nano-oxides with the small sizes (<10 nm) could sequester the helium into several (1–3) tiny bubbles to inhibit bubbles accumulating and coarsening both in elevated temperatures and high helium concentration. Additionally, grain boundaries demonstrated the weaker ability than nano-oxides to disperse helium bubbles, when compared helium irradiation resistance in ODS FeCrAl alloys and FeCrAl alloys. Eventually, ODS FeCrAl alloys in this study revealed the superior helium irradiation resistance than other studied alloys, making it promising to be one of the potential structural materials applicated in advanced nuclear reactors.

Effect of microstructure and local stress on small crack initiation and propagation paths in coarse‐grained FeCrAl alloys

AbstractThis study investigated the high cycle fatigue crack initiation and propagation behaviors in coarse‐grained FeCrAl alloys, with a focus on the relationship between the path of small cracks and the local microstructure. The results show that the small cracks initiate along the {100} slip plane due to the synthetical influence of local microstructure and shear stress. The initiation zone forms inclined areas with layered fine‐grain areas caused by cyclic shear stress. Quantitative analysis of the stress intensity factor (SIF) indicates that in the realm of mixed‐mode loading, the SIF of Mode I takes the lead in determining the intensity of stress and strain fields at the crack tip. As the crack propagates further, the SIF steadily increases. Once it reaches a specific threshold, the propagation mode within a coarse grain shifts from Mode II to Mode I. Moreover, fatigue life increases with a tendency for decreased SIF during crack deflection.

Mechanical and corrosion properties of nano ZrC reinforced FeCrAl alloys

Purpose The purpose of this study is to investigate the effect of nano ZrC particles on the mechanical and electrochemical corrosion properties of FeCrAl alloys, providing a beneficial reference basis for the development of high-performance carbide reinforced FeCrAl alloys with good mechanical and corrosion properties in the future. Design/methodology/approach Nano ZrC reinforced FeCrAl alloys were prepared by mechanical alloying and spark plasma sintering. Phases composition, tensile fractography, corrosion morphology and chemical composition of nano ZrC reinforced FeCrAl alloys were analyzed by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Microhardness and tensile properties of nano ZrC reinforced FeCrAl alloys were investigated by mechanical testing machine and Vickers hardness tester. Electrochemical corrosion properties of nano ZrC reinforced FeCrAl alloys were investigated by electrochemical workstation in 3.5 wt.% NaCl solution. Findings The results showed that addition of nano ZrC can effectively improve the mechanical and corrosion properties. However, excessive nano ZrC could decrease the mechanical properties and reduce the corrosion resistance. In all the FeCrAl alloys, FeCrAl–0.6 wt.% ZrC alloy exhibits the optimum mechanical properties with an ultimate tensile strength, elongation and hardness of 990.7 MPa, 24.1% and 335.8 HV1, respectively, and FeCrAl–0.2 wt.% ZrC alloy has a lower corrosion potential (−0.179 V) and corrosion current density (2.099 µA/cm2) and larger pitting potential (0.497 V) than other FeCrAl–ZrC alloys, showing a better corrosion resistance. Originality/value Adding proper nano ZrC particles can effectively improve the mechanical and corrosion properties, while the excessive nano ZrC is harmful to the mechanical and corrosion properties of FeCrAl alloys, which provides an instruction to develop high-performance FeCrAl cladding materials.

The effect of ramp heating on the microstructure and surface chemistry of APMT FeCrAl alloy

The APMT FeCrAl alloy is a cladding candidate for the accident-tolerant fuel (ATF) systems in light water reactors (LWRs) due to its ability to form a protective Al2O3 film on the surface at temperatures greater than 1000 °C. In order to understand the alumina evolution at the early stages of high-temperature exposure in short periods, an APMT tube specimen was heated up rapidly to melting temperature in a flowing 100 % water and steam environment. Using transmission electron microscopy (TEM), the presence of a ∼ 90 nm thick oxide layer was confirmed. The surface chemistry of APMT was characterized layer by layer using X-ray photoelectron spectroscopy (XPS) depth profiling. The results indicated a thin layer composed of oxides and hydroxides of Al, Cr, and Fe with varying proportions at different depths in the oxide layer. The microstructure of the oxide layer was characterized using X-ray diffraction and showed the presence of α-Al2O3 and γ-Fe2O3/Fe3O4 after ramp heating. The results depict the ability of the APMT FeCrAl alloy to form a protective oxide layer in less than a second on exposure to steam under high ramp rates.

New understanding on the critical factors determining stability of passive film on Fe-Cr alloy based on aberration-corrected TEM study

Purpose In recent years, using aberration-corrected transmission electron microscopy, the authors have achieved precisely detecting the structural evolution of passive film as well as its interface zone at atomic scale. The purpose of this paper aims to make a brief review to show the authors’ new understanding and perspective on the issue of critical factors determining stability of passive film of Fe-Cr alloy. Design/methodology/approach The introduction of single crystal enabled the authors to obtain a distinct metal/passive film interface and better characterize the structure of the interface region. The authors use aberration-corrected TEM to conduct cross-sectional observation and directly capture the details across the entire film at a high spatial and energy resolution. Findings Apart from the passive film itself, the interface zone, including metal/film (Me/F) interface and the adjacent metal side, is also the site which is attacked. Accordingly, the nature of the interface zone, such as microstructure, composition and atomic configuration, is one of the critical factors determining the stability of passive film. Originality/value Deciphering the critical factors determining the stability of passive film is of great significance and has been a fundamental issue in corrosion science. Great attention has been paid to the nature of the passive film itself. In contrast, the possible role of the interface between the passive film and the metal is rarely taken into account. Based on the advanced analytical tool with high spatial resolution, the authors have specified the significant role of interface structures on the macro-scale stability of passive film.

Discontinous and continuous dynamic recrystallization of Fe-12.5Cr–4Al–2Mo-0.03Nb-0.08Y alloy during hot deformation

The hot deformation behavior and microstructure of Fe-12.5Cr–4Al–2Mo-0.03Nb-0.08Y alloy were studied under conditions of 800 °C–1200 °C/0.01 s−1∼10 s−1 using a combination of hot compression experiments, EBSD and TEM characterization. The results indicate that the deformation temperature and strain rate have a significant impact on the flow stress and microstructure of the alloy, especially the deformation temperature. The constitutive equation constructed in this paper can reflect the hot deformation process and accurately calculate flow stress value of the FeCrAl alloy in corresponding states. The FeCrAl alloy undergoes DRV and DRX during hot deformation. DDRX grains distributed along the original grain boundaries mainly formed at low deformation temperatures and high strain rates, while CDRX grains mainly formed at high deformation temperatures and low strain rates and characterized by the internal distribution of the original grains. Moreover, the orientation of DDRX grains is mostly different from that of adjacent original grains, while the orientation of CDRX grains is relatively close to that of the original grains.

High-temperature steam oxidation study of irradiated FeCrAl defueled specimens

Post irradiation examinations (PIE) were performed on irradiated iron-chromium-aluminum (FeCrAl) specimens. These FeCrAl specimens were fabricated at the US Department of Energy's Oak Ridge National Laboratory (ORNL). The experimental setup involved subjecting FeCrAl cladding, along with UO2 pellets, to irradiation in the Idaho National Laboratory Advanced Test Reactor (ATR). In parallel, the FeCrAl alloy tubing without UO2 pellets was irradiated at ORNL's High Flux Isotope Reactor (HFIR). After irradiation, the ATR-irradiated rodlet was transported to an ORNL hot cell, where it was sectioned into multiple samples for the PIE and severe-accident testing. The sectioning process revealed that the fuel was not bonded to the cladding and could be easily detached from sectioned cladding slices. Microstructural analysis of the fuel cross sections demonstrated no significant interaction between the fuel and the cladding. Additionally, high-temperature steam oxidation tests on defueled cladding segments showed minimal oxygen uptake even at 1200 °C. The ATR-irradiated specimens began to exhibit signs of enhanced oxidation upon reaching a temperature of 1300 °C. Furthermore, enhanced oxidation was observed on the inner surface of the ATR-irradiated FeCrAl specimen, which had been subjected to 1300 °C for a duration of 1 min. By contrast, high-temperature steam oxidation experiments indicated that the HFIR-irradiated FeCrAl cladding provided good thermal stability when exposed to 1300 °C for up to 4 h. Comparative analysis encompassing the oxidation behavior of the ATR-irradiated fueled FeCrAl, HFIR-irradiated unfueled FeCrAl, and unirradiated FeCrAl suggests that the fuel–cladding interaction, although not visible via standard microscale electron microscopy measurements, may accelerate the deterioration of FeCrAl cladding in beyond-design-basis accident scenarios.

Grain structure and intermetallic evolution of Nb-containing FeCrAl alloy treated by cryogenic laser quenching

Thin-walled FeCrAl alloy tubes emerge as significant contenders for substituting zirconium alloy cladding tubes. Enhancing the surface strength and corrosion resistance of these tubes is essential for their industrial utilization. Cryogenic laser treatment emerges as a promising technique for enhancing the surface corrosion resistance of Nb-containing FeCrAl alloy. A comprehensive investigation was carried out to study the evolution of solidification structure in a Nb-containing FeCrAl alloy subsequent to cryogenic laser quenching, with the intention of augmenting the mechanical characteristics of the alloy surface. A refined composite structure, consisting of planar-columnar-equiaxed crystals, was identified on the surface of the sample. We have made observation of an amorphous precipitated phase, which is recognized as the intermediate phase found between the Laves phase and ferrite phase. The presence of amorphous precipitates exerts a beneficial influence on the surface hardness and strength of the alloy. Our study delves into the exploration of the causes for the amorphous phase and conducts an analysis of the structural evolution during its recrystallization.

Comparing the oxidation of FeCrAl alloys in isothermal and cyclic high-temperature steam environments

This study investigates the oxidation behavior of FeCrAl (APMT and C26M) alloys in high-temperature steam at 1000 °C using isothermal and cyclic conditions. The FeCrAl alloys exhibited a fully ferritic body-centered cubic structure with no significant phase transformation, and the manufacturing procedures of the alloys resulted in varying grain sizes and porosity. In the 5 h isothermal test, the protective alumina film provided corrosion resistance without cracking in the high-temperature steam environment. However, thermal cycling seemed to compromise passivity; expansion and contraction during heating and cooling allowed oxygen penetration into the underlying metal in local regions, leading to transgranular crack-like features. Spallation of oxides was also observed. Post-exposure analysis of the oxide film formed on the FeCrAl alloys revealed two distinct oxide layers on the alloy: an outer Al-rich oxide film and an inner mixed oxide enriched in Cr and Fe.